Covered rolls are used industrially in demanding environments where they are subjected to high dynamic loads and temperatures. For example, in a typical paper mill, large numbers of rolls are used not only for transporting the web sheet which becomes paper, but also for processing the web itself into finished paper. These rolls are precision elements of the system which are precisely balanced with surfaces that must be maintained at specific configurations.
One type of roll that is particularly subjected to high dynamic loads, is a calender roll. Calendering is employed to improve the smoothness, gloss, printability and thickness of the paper. The calendering section of a paper machine, is a section where the rolls themselves contribute to the manufacturing or processing of the paper rather than merely transporting the web through the machine.
In order to function properly, calender rolls must have extremely hard surfaces. For example, typically, the calender rolls are covered with a thermoset resin having a Shore D hardness within the range of 84-95 and an elastic modulus within the range of 1,000-10,000 MPa. Most commonly, epoxy resins are used to cover calender rolls because epoxy resins form extremely hard surfaces. Thermoset resins, such as, epoxy resins with characteristics suitable for forming the surfaces of calender rolls are cured at relatively high temperatures. Currently, such resins are cured at temperatures in the range of 100-150.degree. C.
It is well known that the higher the curing temperature for heat resistant thermoset resin systems, the higher will be the thermal resistance of the resulting cover. Furthermore, present day demands of the paper mill require rolls, particularly calender rolls, with higher thermal resistances. Thus, it would be desirable to produce covers for such rolls which can be cured at 150-200.degree. C. However, prior to the present invention, curing at such high temperatures caused so much stress that the cover tended to crack, rendering it unusable. A discussion of the physical chemistry of such a roll cover can be found in a paper entitled, "The Role Of Composite Roll Covers In Soft And Super Calendering," J. A. Paasonen, presented at the 46eme Congres Annuel Atip, Grenoble Atria World Trade Center Europole, Oct. 20-22, 1993, the teachings of which are incorporated herein by reference.
Indeed, one important challenge to the manufacture of roll covers is to develop roll covers that can withstand the high residual stresses caused during manufacturing. Problems from residual stresses are most significant in the harder (higher stiffness) compounds and often result in cracking, delamination, and edge lifting.
Residual stresses not only promote the undesirable cracking and/or edge lifting tendency of the cover, but they often cause premature local failure or shorter than desired life cycles. This is especially true for high performance, hard polymeric roll coverings where the basic approach has been to tolerate a production level of residual stresses that is still acceptable from a products performance standpoint. Therefore, there is a need to develop methods of roll cover construction that reduce residual stresses in the product.
Consideration of residual stresses is especially critical during the manufacture of the roll cover. In particular, heating and curing processes must be given careful consideration as these conditions are the most significant factors in the development of such stresses. Residual stresses develop in polymer based covers as a result of the mismatch in thermal shrinkage properties between and/or among the cover materials and the core materials and from chemical shrinkage. Polymers, the material of the cover, typically have a coefficient of thermal expansion that is an order of magnitude greater than that of steel, the material of the core.
A suggestion to alleviate stresses caused by processing covered rolls is to produce a cover as a finished product and bond the fully cured cover to a core structure. This can be accomplished by wrapping a cover (topstock) over a mold, demolding and bonding the cover to a core structure at a lower temperature level than the cover cure temperature, or by casting the cover separately and bonding it to a metal core at a lower temperature than the casting temperature. Thus, the thermal stresses that would arise between the cover and the core from cooling down the cover from the cure temperature would be reduced. Although, adhesives are available, some adhesives exhibit poor bonding strengths when the roll is subjected to industrial applications. In general, adhesives that cure at high temperatures are required for high temperature performance. However, subjecting the core to high temperature bonding conditions results in those stresses that were avoided by separately producing the cover.
In addition, manufacturing costs would be raised by the necessity of having to produce the cover first as a separate cylindrical structure, and then, fitting it over a roll core at a lower processing temperature than was required for processing the cover. These casting methods require that an open cavity be created between the cover and the roll core which necessitate multiple process steps and the use of inner mandrels. Even if the cover is separately manufactured via a centrifugal casting method, additional costs and steps are required for an outer mold.
Another possible solution is to develop a cover material having a thermal shrinkage as close to the metallic core as possible. While composite structures may be developed with the expansion coefficients tailored to match the metal core, such methods are expensive and may not produce the desired thermomechanical response for certain industrial applications. Thus, the need exists to develop methods to reduce the residual stress levels in current production materials.